Eli,
Am 12.03.2016 um 08:08 schrieb Eli Cooper:
> Hi Richard,
>
> On 2016/3/10 4:44, Richard Weinberger wrote:
>> Hmm, this needs rework. Having everything on the stack is not good.
>
> Okay, I'll rework the functions whose stack size is greater than the
> warning threshold by using kmalloc.
I fear it is not that easy. Having a kmalloc() per context switch would
be every expensive. Even for UML.
>> Can you also create a selftest such that this bug cannot happen again?
>
> It seems that instead of writing a self-test showing this problem cannot
> happen again, I wrote a test that manifested another bug that is not
> directly related to my patch.
>
> Without applying my patch, the current UML should support XMM registers
> because those are covered by _fpstate and PTRACE_GETFPREGS. But it
> seemed that XMM registers are not restored after the signal handler returns.
>
> In the following quick test, the main loop should run indefinitely
> despite XMM registers are modified by the signal handler. But in UML,
> the loop breaks randomly within a minute or two, showing that the
> registers are corrupted. So far I haven't found the cause. Any hints?
Meh. :(
Can you figure out whether the issue depends on the host kernel? i.e. try
something older
and Linus' tree.
UML is a heavy user of ptrace(), maybe the recent FPU cleanup on x86 broke
something.
Thanks,
//richard
> Thanks,
> Eli
>
> ---
> /* test if signal handling preserves XMM registers */
> #include <stdio.h>
> #include <unistd.h>
> #include <signal.h>
>
> int count;
>
> void sighandler(int signum)
> {
> count++;
>
> /* alarm(1) without calling libc */
> asm("mov $0x1,%rdi");
> asm("mov $0x25,%rax");
> asm("syscall");
>
> asm("movq $0xdeadbeef,%r11");
> /* the following two instructions
> * modify xmm0 and xmm1 registers */
> asm("vmovq %r11,%xmm0");
> asm("vmovq %r11,%xmm1");
> }
>
> int main()
> {
> struct sigaction act;
> double a = 3.14159, b = 2.71828;
>
> act.sa_handler = sighandler;
> act.sa_flags = 0;
> sigemptyset(&act.sa_mask);
> sigaction(SIGALRM, &act, NULL);
>
> alarm(1);
>
> /* this loop should run indefinitely */
> while (a + b == a + b) ;
>
> printf("count = %d\n", count);
> return 1;
> }
>
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